The container control sends a WM_CONTROL (in Container Controls) message with the CN_DROPNOTIFY notification code to its owner when a pickup set is dropped over the container.

param1
     USHORT            id                /*  Container control ID. */
     USHORT            CN_DROPNOTIFY     /*  Notification code. */

param2
     PCNRLAZYDRAGINFO  pCnrLazyDragInfo  /*  Pointer to the CNRLAZYDRAGINFO structure. */